public class Bird extends Animal {
    public int speed;
    public String fly(){
        return "翱翔天空";
    }
    public Bird(){
        System.out.println("You build an animal.");
    }
    public Bird(int age, int weight){
        System.out.println(String.format("You build an %d years old eagle with %dkg weight.", age, weight));
    }
}
